Glass or Plastic: The Question for SkylightsJanuary 29, 2010
Architects have increasingly placed more emphasis on daylighting in commercial buildings. One reason stems from the results of three different studies recently conducted by the Heschong Mahone Group Inc., a research house in Fair Oaks, Calif., which have shown that natural daylight enhances student productivity.1 Taking this a step further and applying these results to the working world, one can presume that daylight will also lead to higher sales in retail stores and improved employee productivity in office buildings. It is all about the environment, and a major part of daylighting in buildings comes from skylights.
With the increasing popularity of skylights, more architects, building owners and specifiers are eager to learn about the performance, costs and energy efficiency of different glazing options used on skylights.
Glazing techniques vary according to the demands of each project. Some designs require a clear view through the glazed surface, while others seek to promote natural, daylit illumination of a particular area. Both glass and plastic products can provide modern design while complying with stringent hurricane-resistance and storm protection standards.
Basic Glazing Materials
The first option that often comes to mind is glass. Although commonly a more expensive option, glass is durable, long lasting and highly scratch-resistant. Various types of glass provide different degrees of energy efficiency. Insulating glass units, for example, often use laminated or tempered glass, a spacer and a second sheet of glass to provide a sealed, moisture-free option. When diffuse light is required, white coated laminated layers with high haze value are available for many standard glass-glazed units.
Glass with low-emissivity coatings, with or without the white translucent coating, can be beneficial in the southern U.S. to reduce heat gain (lower solar heat gain coefficient) and advantageous in the northern U.S. to reduce heat loss (better U-factor). Laminated glass, comprised of two sheets of glass with a polyvinyl butyral plastic interlayer (or like material), is used in most blast-resistant and hurricane-resistant glazing, and it is given preference in codes for glass skylight safety without the need for retaining screening. The glass may fracture, but fragments remain bonded to the interlayer.
Though glass is less formable than plastic, it can be curved or shaped. Annealed glass is not as strong as heat-treated glass, but it can be easily cut. Both heat-strengthened and tempered glasses are approximately two and four times stronger than annealed glass, respectively, but cannot be cut. Tempered glass meets the safety glazing codes because it fractures into small fragments when broken. Glass can also be hermetically sealed to retain gas infills.
Acrylic plastic was first developed in 1928 and was brought to market in 1933. Acrylic plastic is used in many standard skylights and is the most formable of the skylight glazing options. Acrylic sheet, manufactured by either a cast or extrusion process, can be easily cut. Modified acrylic plastics are available to achieve impact resistance performance. Some acrylic plastics are transparent to ultraviolet radiation (UV) and therefore weather extremely well. UV absorbers, however, are typically added to offer UV protection to materials struck by sunlight passing through acrylic sheet. Acrylic sheet used in skylights is offered in transparent or translucent versions. Transparent acrylics often have a bronze or gray tint, whereas translucent choices usually come in white or patterns. The prismatic pattern seen in fluorescent light covers is most familiar.
Polycarbonate was first discovered in 1953 both in the United States and Germany and was brought to market in 1959 as one of the first "space age" materials. Polycarbonate is very durable, highly impact-resistant and can be easily cut. Polycarbonate also can be thermoformed but at higher temperatures and more energy use than acrylic and must also be pre-dried before thermoforming. Manufactured by an extrusion process, polycarbonate comes in monolithic sheets and multi-wall sheets.
The multi-wall sheets are extruded with multiple layers, and the honeycomb effect creates air spaces for more energy efficiency. Multi-wall sheets filled with aerogel, a nanoporous silican dioxide material also known as “frozen smoke,” are lightweight and can be cut to size. They also provide improved insulating value as well as acoustic value so as to greatly minimize noise that typically comes through a skylight. Panels utilizing this aerogel technology provide excellent light diffusion, helping to eliminate glare and hot spots.
Naturally transparent, polycarbonate transmits almost as much light as glass, is heat resistant and has excellent dimensional and color stability. However, in some cases, they offer only fair chemical resistance and have poor abrasion resistance. Polycarbonate Plastic sheets are available with a UV-light-absorbing layer, offering protection from UV wavelengths to help establish long-lasting product strength, color and clarity. Abrasion properties can be enhanced through the use of topcoatings applied to the sheet.
Copolyester plastics were first discovered in 1941. Copolyester offers another glazing alternative, providing transparent clarity and a high-gloss finish. Copolyester plastic sheets are also available with a UV-light-absorbing layer providing protection from UV wavelengths and ensuring long lasting product strength, color and clarity. Available in smooth, matte and textured surface finishes in both transparent and translucent forms, copolyester plastics provide durability and flexibility in design. Copolyesters are designed to provide a unique balance of toughness and stiffness to provide impact resistance performance. Copolyesters may also be chemical resistant, depending on the chemicals used, and have abrasion resistance similar to polycarbonate. Scratches are easily repaired with a hot air gun or the sheet may be topcoated with an abrasion resistant layer to prevent scratches. With excellent clarity and low haze, copolyester is easy to thermoform and requires less energy and lower temperatures than acrylic or polycarbonate. Sheets of copolyester do not need to be pre-dried before thermoforming.
Fiberglass is a translucent option that can be monolithic, domed, flat or various other profiles and can feature two pieces with an air space for insulation. The pieces create a panel that provides excellent light diffusion. Fiberglass alone is not formable, but can be shaped and molded inexpensively and can be bonded to I-beams and often is used to form structural, translucent panels. Such translucent panels have high insulating values and feature a variety of grid patterns and color combinations. Fiberglass enables effects that cannot be duplicated with other materials.
Translucent fiberglass panels are virtually shatterproof and impact-resistant, yet lightweight. UV protection is often integral to the product and protects these panels from weather damage, fiber bloom and discoloration.
What Professionals Say
David Moss, an independent architect and roof consultant, based in Lakewood Ranch, Fla., works on projects along the East Coast from Florida to New York. Due to the wide variety of climates, Moss’ clients voice diverse concerns. For instance, in Florida, where most schools do not have ventilation systems, unwanted heat buildup from skylights has become a major concern. While most small skylights are made of acrylic or polycarbonate, according to Moss, the use of hurricane-resistant glass has also become very popular in Florida.
By choosing the right material, Moss creates dramatic changes in lighting. For instance, he recommended updating six 20-by-120-foot skylights at Genesee Community College, Batavia, N.Y. To replace the translucent, bronze-tinted panes, he specified a clear, white, translucent material. “Other pluses can come from choosing the right infill,” Moss says. “Laminated glass, acrylic and polycarbonate absorb UV rays, reducing fading of furniture, fabrics and carpet.”
“Architects and building owners specify skylight infill based on quality, aesthetics and performance,” says Michael Nielsen, president of W.S. Nielsen Skylight Systems in Alpharetta, Ga. From Nielsen’s perspective, “glass is class.”
Furthermore, although the most expensive, glass has, by far, the longest life span. Nielsen says that most of the glass used today is either laminated or heat-treated. Plastics cost less than glass, and are often specified for different colors or back lighting. “Plastics also come into the picture when lighter loading is an issue,” Nielsen says.
Translucent panels are middle-of-the-road in cost and are specified for better insulation performance. The insulation factor makes them popular in the north. In the south, Nielsen says, light transmission and heat-gain management are important when specifying a glazing infill for skylights.
Walter Scarborough, director of specifications for HKS Inc. in Dallas, sees the various glazing products as “different opportunities to give a skylight an aesthetic character,” ranging from the refined elegance of glass tinted to complement an architectural design,
to fiberglass only used for utilitarian purposes. Skylights instill drama in interior spaces, Scarborough says, and the glazing material that will make the strongest statement should be selected.
“Glazed skylights are a design feature whose creative potential is virtually untapped and rarely achieved,” Scarborough says. “I recently saw photographs of a skylight that was a tent-like structure, complete with wooden poles at the center, over a twisting-and-turning hospital walkway in which the glass panels were glazed into the framing in a shingled fashion.” His recommendation is for architects to push the envelope away from the traditional shed shape skylights with clear glass.
Whatever the aesthetic and performance needs, there is a glazing material to fill those needs. From considerations of weather or light transmission, the right material for the right job can produce outstanding results across a multitude of criteria.
Originally posted March 2009